Perforation refers to a hole punched in the casing or liner of an oil well to connect a wellbore to a reservoir. In cased hole completions, the wellbore is drilled either at or past a section of the formation desired for production. Casing or a liner is used to separate the formation from the wellbore that is often cemented into the producing reservoir. The final stage of the completion process involves running in perforating gun assemblies down to the desired depth and/or orientation and firing them to perforate the casing, liner, cement, and/or the producing formation. Perforating gun assemblies may use steel bullets or shaped explosive charges, which, when detonated, produce extremely fast jets of gasses that blow the perforations into the casing, cement, and producing formation. A typical perforating gun assembly can carry many dozens of charges and may include a string of shaped charges.
Today, perforating gun assemblies can be conveyed on wireline, slickline, coil tubing or tubing (production tubing, drill pipe, etc). The perforating service methods, features and capabilities depend on the type of conveyance. For example, wireline perforating allows an accurate real-time depth correlation by running gamma ray (“GR”) and casing collar locator (“CCL”) measurement tools with the perforating gun assemblies along with surface to downhole telemetry via wireline cable. Wireline perforating can use a fast and simple gun initiation via wireline cable, to initiate the perforating gun assembly. However, wireline cable strength (or tensile) rating limits the length and number of perforating gun assemblies. In addition, wireline deployment in highly deviated or horizontal wells requires the use of downhole tractor or pumping down technique that may still be limited in terms of gun length/weight or require multiple trips in the hole. Typically wireline guns are used for overbalanced or balanced perforating (i.e. pressure in the well is equal or higher than formation pressure), as underbalance perforating typically induces well to flow and poses a risk of guns and the cable being blown uphole or requires an anchoring mechanism to keep them in place.
Slickline is wireline having a high strength to weight ratio. However, no communication with the surface is possible with slickline. Instead, a special firing head is used that senses a timed up/down motion of the slick line that constitutes a unique firing command to initiate the guns. Alternatively, other firing head designs may use a mechanism on the gun that arms the charges upon reaching a certain temperature and pressure. A timer will then fire the perforation assembly following a set interval. The depth correlation for Slickline perforation is typically done via separate run with GR/CCL tool in memory mode and marking Slickline reference point at surface for applying depth offset determined from the GR/CCL log. The main advantage of the Slickline is low cost and small footprint of the Slickline unit thus simplifying deployment logistics, for example on the small production platform. In all other ways like tensile rating and control/feedback of perforating events it is inferior to Wireline and TCP perforating.
New wireline and TCP perforating techniques with dynamic underbalance (PURE system) have been introduced to facilitate removing the explosion debris from the perforating tunnel, thus improving the well producability (reducing skin effects). However, all other limitations of the wireline and TCP deployments described above still remain.
In cases where a long gun string is too heavy for wireline cable, or when flowing the well immediately after the perforation to put the well on production (in the case of permanent completion), or to test or clean up the producing interval after perforating, the guns are run on the tubing. This is the essence of tubing conveyed perforating (“TCP”). Highly deviated and horizontal wells may also require the use of TCP to gain access to the desired perforating depth. If flowing the well is required after the perforation, it typically is accomplished by establishing the pressure in the tubing interior diameter (hereinafter “ID”) lower than formation pressure before the perforating. Known methods for accomplishing this include placing low weight fluid or gas in the tubing ID and sealing it off from the well's annulus with a packer. Often, flow (test) valves and circulating (tubing ID-OD) valves are run with the string to facilitate well flow and safety control, placement of fluid cushions, circulating out well effluent, etc.
TCP techniques enable perforating very long intervals in one run. For example, some TCP strings have exceeded 8,000 ft in length. TCP also facilitates running large guns and using high underbalance. Without having to kill the well, TCP strings can be retrieved (shoot and pull) or left as part of the permanent completion (integrated TCP).
Typically, to initiate the TCP guns hydraulic/mechanical tools and percussion detonators are used, triggered by applying additional pressure to the well or dropping the bar from the surface, etc. A new technology, eFire firing head, was introduced in the last decade. The eFire is a battery powered microprocessor controlled tool with an electrical initiator that can be triggered by lower level signals (typically pressure). Additionally, the initiation energy to set off the eFire detonator is provided by an on-board battery. While the eFire offers more flexibility and efficiency to many TCP operations, all TCP initiation methods are inferior to WL perforating that is done via simple entering of the perforating instructions in the surface computer. Moreover, TCP operations typically require a separate GR/CCL run on WL for depth correlation and proper placement of the perforating gun assemblies in the producing interval before initiation.
Another disadvantage of the TCP is a lack of data from downhole tools confirming the perforating operation or providing any downhole measurements before the string is pulled out of the hole. Typically, TCP operations utilize redundant firing heads or methods, but if the well does not flow to the surface it is often hard to confirm whether the guns were fired or the well did not produce as expected. Beyond the confirmation that the job objectives were met, retrieving unspent and potentially armed gun string from the well is a huge risk and an improved system and method is needed.
Thus, tubing conveyed methods and tools are difficult to control as there is no real time communication, no electrical signals, and operation is usually performed via simple pressure levels or timed pressure pulse instructions or by pure mechanical means (e.g. drop bar). For accurate depth placement, tubing methods typically require wireline run in conjunction with deployment of tubing conveyed perforating guns, which are often done in separate runs, increasing time and cost to perforate a casing. Wireline methods and tools for perforation also have limited applications. For example, wireline cannot employ a large number or great length of perforating gun assemblies because of the heavy weight of the gun. Wireline is also usually limited to short intervals and not too deviated wells. Therefore there is a need for improved methods and systems for perforating a wellbore casing.